1. Field of the Invention
The present disclosure relates to wireless data communication in general, and, in particular, to wireless data communication systems using switched-beam or other directional antenna technology, and the computation of a steering metric (SM) to enable optimization of antenna position (antenna pointing direction).
2. Description of the Related Art
Wireless data communications systems enable data transmission among two or more network elements. An example is a wireless local-area network (WLAN) system, widely used for connecting network elements in homes and offices, based on IEEE standard 802.11x (data rates from 6 to 54 Mbps). Operating range in a wireless system typically decreases with increasing data rate, for a given transmit power (which is often limited by law). Typical wireless network elements such as a WLAN access point (AP) use omni-directional antennas for receiving and transmitting data because network elements typically have no knowledge of the location of other network elements desiring a wireless connection.
Directional antennas have the desirable property of increasing the gain and hence communication range, by focusing the transmitted or received energy into a narrower beam. Many known approaches for generating such directional beams are used, including switched antennas, phased arrays of antenna elements, and others. One such approach is known as switched-beam antenna. The switched-beam antenna has plurality of typically identical beams, each covering an angular range with some fraction of 360 degrees, and oriented to direct the energy of the beam in a different direction. For example, a 6-beam antenna has six beams approximately 60 degrees wide, each beam typically oriented 60 degrees from the other, to provide full 360 degree coverage. Such antenna provides improved gain compared with an omni-directional antenna, and also provide increased transmit and receive range.
Application of such directional antenna in a wireless data communication system typically requires an automated means of determining the optimal antenna position to use for communication with other network elements at a given time. The “antenna position” refers to the angular position of a directional beam, or the omni-directional pattern. Typically, each of the many given antenna positions is tried to determine which position gives the best results. Each trial evaluates a parameter directly or indirectly indicative of the quality of data and compares the result for each position to determine the optimal position to use for communication. One widely-used such parameter is received signal strength indication (RSSI), which is typically available as analog or digital data from the automatic gain control (AGC) circuit in the network element receiver.
At lower data rates, and/or in an environment with minimum multipath, the use of RSSI to determine optimal antenna position can be quite effective. The determination of RSSI, as the antenna position changes, is typically simpler and quicker than the determination of packet error rate (PER). As a result, training time and data overhead is reduced by using RSSI at low data rates.
However, at higher data rates, multipath effects the quality of the received data (as measured by packet error rate PER) more that than RSSI. The PER might be significantly better using an antenna position having a lower-than-peak RSSI. Many wireless communication systems support widely-varying data rates. For example, WLAN standard 802.11g provides for data rates typically ranging from 6 to 54 Mbps. Lower rates are used in difficult transmission path conditions (long distance, high multi-path, interference from other network elements), while higher rates are used in better conditions. Use of only PER or RSSI to determine optimum antenna position over such a wide range of bandwidth is non-optimal. Therefore, a system and method is needed to effectively optimize antenna positioning when using a directional antenna wireless communication system while minimizing overhead (data bits not directly carrying user information) with a relatively shorter training time than traditionally used.